This invention relates to bonding fluoroelastomers to metal. More particularly, the invention relates to a method of directly bonding fluoroelastomers to metal by non-adhesive means, i.e., without the use of a bonding agent.
The fluoroelastomers of this invention are typically relatively chemically inert, thermally stable polymers, owing primarily to the strength of the carbon-fluorine bonds present in the molecule. Because of the thermal stability of these molecules, the fluoroelastomers are desirable in many applications which require elastomeric materials which will withstand high bulk temperature moduli. However, a great number of these applications, particularly in mechanical fields, require the elastomer to be used in conjunction with stiffening, or backup members, most often fabricated from metal, and hence require a method of bonding the elastomer to the metal member which will provide a bond capable of withstanding high temperatures and sustained or heavy loading. Particular examples include clutch plates and brake linings, where high friction coefficients result in very high ambient temperatures; many other examples will be apparent.
Unfortunately, the inherent properties of the fluoroelastomers, in particular chemical inertness, have made it difficult and, in some instances, impossible to effectively adhere these fluoroelastomers to a metal, especially where high shear or impact strength is required in the finished fluoroelastomer-metal laminate, or where the finished laminate will be exposed to high ambient temperatures. The problem of bonding the ultra-high temperature thermally stable fluoroelastomers has been particularly troublesome.
One particular problem which exists with obtaining adequate bonding of fluoroelastomer to metals while at the same molding the fluoroelastomer to a desired shape and curing it in that shape has been introduced by the fact that curing of the fluoroelastomer and bonding of the fluoroelastomer to metals generally occur at very different rates. If the curing is much faster than the bonding, then inadequate bonding results. On the other hand, if the bonding is mush faster than the curing, then the laminate must be retained in the mold long after the bonding is complete or the laminate will not be properly formed and a gummy mess will be all that is removed from the mold. Long times within the mold are clearly undesirable from a practical view point since the yield of finished laminates then becomes limited by the mold time.
Prior art methods which have been employed for bonding some types of fluoroelastomers to metal have most commonly involved chemical bonding of the fluoroelastomer to the metal substrate with an adhesive bonding agent. These methods have limited application, however, since with both structural and nonstructural adhesives, the bond achieved is not generally capable of withstanding sustained or heavy loading, or high ambient temperatures. Further, the adhesive bonding agent is highly subject to scuffing and wiping during injection and transfer molding of the product, which deactivates or destroys this agent, and therefore molding techniques and mold configurations for fluoroelastomer-metal laminates have theretofore been highly restricted.